Slag use in continuous casting of steel

ABSTRACT

A process of continuous casting of steel in which a layer of a molten synthetic slag about one-half to two inches thick is maintained on the surface of a pool of molten steel in the upper end of the continuous casting mold, and said synthetic slag being a mixture of oxides and fluorides wherein said oxides and fluorides are combined in said slag to provide said slag with a plastic deformation point between about 1,100* F and about 1,700* F in accordance with the following equation: Plastic Deformation (Point (* F.)) 1,540- 1 wt. % (K20+ Na20)14 wt. % (B203)- 54 wt. % (LiF+ Li2O)+ 10 wt. % (CaO+ CaF2)+ 1 wt. % (SiO2), said oxide and fluorides also being combined in said slag to provide said slag with a flowidity between about 2 inches and about 16 inches in accordance with the following equation: Flowidity (inches) 9.4+ 0.02 wt. % (K20+ Na20)+ 0.05 wt. % (B203)+ 0.10 wt. % (CaO+ CaF2)+ 0.32 wt. % (LiF+ Li20)- 0.20 wt. % (SiO2), and said slag having an alumina solubility in excess of 20 percent by wt.

- FLOWlDlT C Umted States Patent 1151 3,704,744 Halley et al. 1 Dec. 5, 1972 1 SLAG USE IN CONTINUOUS CASTING Primary Examiner-J. Spencer Overholser OF STEEL Assistant ExaminerV. K. Rising [72] Inventors: James W. Halley, Chesterton, lnd.; Attorney-George Hlbben Donald E. Grimes, Lasing, lll.; Norman T. Mills, Highland, lnd.; Krish- [57] ABSTRACT na Rao Yalamanchili, N Yo k, A process of continuous casting of steel in which a NY, layer of a molten synthetic slag about one-half to two inches thick is maintained on the surface of a pool of [73] Asslgnee' Inland Steel Company Chlcago molten steel in the upper end of the continuous cast- [22] Fil d; O t, 22, 1971 ing mold, and said synthetic slag being a mixture of oxides and fluorides wherein said oxides and fluorides App! N05 191601 are combined in said slag to provide said slag with a plastic deformation point between about 1,100F and Related Application Data about 1,700F in accordance with the following equa- [60] Division of Ser. No. 52,750, July 6, 1970, Pat. No. tion:

3,649,249, which is a continuation-in-part of Ser. No. plastic Deformation 7 4 b. 0.

64 Fe 2 ounce) =1540-1 wt.%(l(zO+Na O) [52 US. Cl. ..l64/82, 164/4, 164/73, 14 (32.03)

148/26, 75/94, 75/96 54 (up 51 1m. (:1. ..B22d 11/00 +10 (.CaO+CaF2) [58 Field 61 Search ..75/96, 94, 68, 21; 148/26;

164/73 82, 52 said oxide and fluorides also being combined in said slag to provide said slag with a flowidity between [56] References Cited about 2 inches and about 16 inches in accordance with the following equation:

UNITED STATES PATENTS Flowidity (inches) =9.4-10.02 wt. (K,0+Na,0) 3,649,249 3/1972 Halley 6161. ..75/96 13 ggzggggg 3,642,052 2/1972 Schrewe et al.... ..l64/82 +0.32 wt1%(LiF+Li 0) 3,318,363 5/1967 GOSS ..164/82 (Sim)- 2,825,947 3/1958 Goss ..l64/73 and i Slag having an alumina Solubility in excess f Mlllel' et a] percent wt 10 Claims, 2 Drawing Figures l800 k. z B 1. 8 1700- Y 0. z [600- O t 1500 I400- 25 1300- E k U.) [200- Q U 1100- A F D locov,

)Ofi "i IN INCHES SLAG USE IN CONTINUOUS CASTING OF STEEL RELATED APPLICATIONS This application is a divisional application of copending application Ser. No. 52,750, filed July 6, 1970, now US. Pat. No. 3,649,249, which was a continuation-in-part of application Ser. No. 7,464, filed Feb. 2, 1970.

The present invention relates generally to continuous casting of steels and more particularly to improved synthetic slag compositions for use in the continuous casting of steels.

In the continuous casting of steels, it has heretofore been found that improved casting results are obtained by providing on the upper surface of the molten steel in a continuous casting mold a protective layer of synthetic slag. Many different compositions have been proposed for use as the synthetic slag in a continuous casting mold, including blast furnace slag, window glass, bottle glass, and sodium silicate glass which have heretofore been referred to as low viscosity slags. Borax has also been proposed for use in the continuous casting of steel as a low viscosity slag.

The prior art continuous casting slags which have been proposed for use in the continuous casting of steel possess one or more desirable properties, such as forming a protective sheath between the molten steel in the mold and the mold walls, reducing friction between the steel casting and the mold wall, improving heat transfer between the steel casting and the mold wall, keeping the surface of the molten steel insulated against excessive heat loss, and preventing oxidation of the molten steel surface.

In addition to the previously disclosed desirable properties, however, a satisfactory slag composition for the continuous casting of steel should also have good chemical and thermal stability while in contact with the molten steel in order to avoid producing objectionable fuming, toxic gases or decomposition products capable of being absorbed by the molten steel and causing objectionable contamination of the steel. The stability of the slag composition is also important where decomposition or vaporization would result in significantly changing the physical and/or chemical properties of the slag at the elevated temperatures which exist in the continuous casting mold.

It has now been found very important that the slag composition also have a high solubility for aluminum oxide which frequently is the cause of surface defects in a steel casting or in the rolled steel sheet materials produced therefrom, particularly when casting aluminum killed low carbon steels which contain relatively large amounts of aluminum oxide.

An additional characteristic which has been found important in a continuous casting slag to be used in casting aluminum killed steel or the like is that the slag possess a combination of plastic deformation point and flowidity" properties along with the high alumina solubility" (See page 13 for definitions of the foregoing terms) which is capable of providing in the continuous casting mold a relatively thick layer of fused slag on the surface of the molten steel pool maintaincd in the continuous casting mold to effect removal of substantial amounts of aluminum oxide from the steel, while at the same time permitting a continuous and uniform flow of slag over the lateral surfaces of the steel casting without consuming excessive amounts of slag during the continuous casting operation.

lt has further been found important that a continuous casting slag for use in casting aluminum killed steel or the like possess characteristics which improve the heat transfer from the continuous steel casting to the water in the secondary cooling zones after the casting is withdrawn from the continuous casting mold.

None of the prior art continuous casting slag compositions possess all of the foregoing properties which have now been found to be extremely important requisites of an effective slag capable of providing continuous steel castings substantially free of surface defects, and particularly from steels containing significant amounts of aluminum oxides.

It is therefore an object of the present invention to provide an improved synthetic slag composition which facilitates the production of high quality steel continuous castings from steels containing aluminum oxides as objectionable inclusion materials.

It is a further object of the present invention to provide an improved synthetic slag composition which can be used effectively in continuous casting operations without fuming or creating other hazardous and objectionable operating conditions.

It is another object of the present invention to provide an improved synthetic slag composition which is substantially chemically and thermally stable when used for the continuous casting of steel.

It is also an object of the present invention to provide an improved synthetic slag composition which has a high solubility for aluminum oxides contained in molten steel.

A further object of the present invention is to provide a synthetic slag composition having plastic deformation point and flowidity properties which provide an improved slag layer for the continuous casting of steel.

A still further object of the present invention is to provide a synthetic slag composition which markedly improves the heat transfer from the continuous steel casting to the water in the secondary cooling zones after the casting is withdrawn from the continuous casting mold.

Still another object of the present invention is to provide an improved process of formulating and preparing a synthetic slag composition for the continuous casting of steel.

Other objects of the present invention and the scope of the present invention will be fully understood by those skilled in the art from the following detailed description and claims when read in conjunction with the accompanying drawing; wherein FIG. 1 is a graphic representation of the relationship between the observed plastic defonnation point and the flowidity exhibited by the slag compositions coming within the scope of the present invention; and

FIG. 2 is a perspective view partially in vertical section of apparatus used for measuring the flowidity of the slag compositions of the present invention.

It has been found that the foregoing objects and other objects of the present invention can be achieved by providing a synthetic slag composition which is substantially non-fuming, and substantially chemically and thermally stable when used for continuously casting steel, which exhibits an alumina solubility in excess of 20 percent by weight, a plastic deformation point between about l,l F and about l,700 F, and a flowidity between about 2 inches and about 16 inches, and in which the batched mixture of the slag components prior to fusion has a chemical composition falling substantially within the following ranges:

SiO to 55% by wt. CaO 0 to 40% by wt. CaF, 5% to 40% by wt.

Na O+K O (either or both) Li- O+LiF (either or both) B 0 5% to 30% by wt.

0.5% to by wt.

- 0 to 30percent by wt. greater than 15% by weight.

It has been further found that the plastic deformation point and the flowidity of the slag composition of the present invention have the empirically determined relationship with the batched chemical composition of the slag expressed by the following equations:

Flowidityfinches) in the above equations the wt. for each designated oxide or fluoride corresponds to the weight per cent in which the indicated oxide or fluoride is present in the batched material before heating to form a uniform melt (i.e. prefused slag).

It will be evident from the foregoing equations and the chemical composition ranges specified for the oxides and fluorides that there can be considerable variation in the proportions in which the oxides and fluorides can be combined and that the compositions of the present invention can have a considerable range in the flowidity and plastic deformation point thereof without departing from the scope of the present invention. FIG. 1 of the accompanying drawing shows graphically the operating limits and the preferred ranges of the plastic deformation point and the flowidity of the slag compositions of the present invention. The rectangular area defined by the points ABCD in FIG. 1 shows the operating limits of the plastic deformation point and flowidity of the slag compositions which have been found to produce improved continuous steel castings in accordance with the present invention, and the area defined by the points ACEF shows the preferred ranges. In the preferred embodiments of the present invention, when the plastic deformation point of the slag composition is near the upper limit of the specified range, the flowidity of the slag preferably should also be in the upper limit of the specified flowidity range; and if the plastic deformation point of the slag composition is in the lower part of the specified range, the flowidity of the slag can be in the lower part of the flowidity range. A slag composition having a relatively low plastic deformation point and a relatively high flowidity within the herein specified ranges produces a satisfactory continuous casting, but the consumption of the slag will be higher than with the preferred embodiments.

The slag composition of the present invention can be prepared from a wide variety of raw materials which contain or which when heated produce the oxides and fluorides in the amounts required to provide the herein specified slag compositions. For example, sodium silicate can be used as a source of silicon dioxide and sodium oxide; window glass can be used as a source of silicon dioxide, sodium oxide and calcium oxide; fluorspar can provide the required calcium fluoride; metal carbonates can be used as the source of the corresponding metal oxides; and borax can be used as the source of the boron trioxide and sodium oxide. Compounds containing water, however, such as calcium hydroxide or hydrated lime should be avoided, because some fluoride would be driven off with the water during the fusion process. If desired, any one of the individual decomposable source of materials, such as sodium carbonate, from which an oxide or a fluoride specified in the formulation is produced on heating can be heated before batching to provide the desired oxide or fluoride. It should also be understood that oxides and fluorides which are equivalent to the herein specified batched chemical composition can be substituted without departing from the disclosed invention. For example, barium oxide, magnesium oxide and strontium oxide can be substituted for part of the calcium oxide. if desired.

When preparing the batched mixture of the several oxide and fluoride containing material from compounds and materials other than the chemically pure ingredients specified, the impurities present therein should be taken into consideration when calculating the amountof oxide or fluoride ingredient required in the batched mixture or slag composition. Likewise, when using a thermally decomposable compound, such as a metal carbonate, as a source of an oxide ingredient, the amount of the decomposable compound used should be adjusted to provide the required amount of oxide in the batched mixture.

When selecting compounds or materials other than the chemically pure oxides and fluorides for preparing the slag compositions of the present invention care should also be used to keep to a minimum extraneous impurities and to avoid particularly including therein impurities which would be detrimental to the steel or the slag composition. Thus, the manganese oxide and iron oxide content of the slag ingredients should be kept as low as possible, because these oxides will oxidize any aluminum or silicon in the steel. Also, oxides of elements which are readily reduced by molten iron, such as oxides of nickel, cobalt, lead, copper, molybdenum, tungsten and phosphorous, should not be included in the slag forming materials, unless it is desired to include such an element in the particular steel being produced for some metallurgical reason.

Particular attention should be given to the alumina content of the raw materials used in preparing the slag composition, since the alumina solubility of the stag composition is reduced by an amount almost directly proportional to the initial alumina content of the slag composition. For example, if the initial slag composition contains 2 percent by weight alumina, the sum of the percentages by weight of the boron trioxide, calcium fluoride and lithium fluoride in the slag composition must be in excess of about 17 percent by weight, or in other words about 2 percent above the lower specified limit of percent required when the initial slag contains negligible amounts of alumina.

In some steel continuous casting operations the boron trioxide content of a slag should not exceed about 2 percent by weight of the slag, since more than 2 percent by weight boron trioxide in the slag can result in increasing the boron content of some steels to a level which is objectionable.

In most steels a small increase in the normal boron content of the steel is not objectionable. However, boron trioxide has a high vapor pressure at the temperature of molten steel in a continuous casting operation, and can be present in a satisfactory synthetic continuous casting slag only in limited amounts. For example, it is an inherent characteristic of a borax slag or a slag having a high boron trioxide content to fume badly, and when used as a slag layer in the continuous casting of steel these slags create very objectionable and commercially intolerable operating conditions. It has been found, however, that when the boron trioxide content does not exceed 30 percent by weight of a slag composition and has the herein specified composition, there is no objectionable fuming of the composition during the continuous casting of steels.

It will be further evident from the herein disclosed equations showing the relationship between plastic deformation point or flowidity and the chemical composition of the slag that the lithium content of the slag, either as lithium fluoride or lithium oxide, has by far the greatest effect on the flowidity and plastic deformation point of the slag composition, and lithium fluoride and lithium oxide when used even in small amounts have a very substantial effect on both the flowidity and the plastic deformation point of the slag.

The slag compositions of the present invention are preferably prepared by mixing in the required amounts the several oxides and the fluorides or the oxide and fluoride containing materials to form the batched material or mixture and heating the mixture to a temperature sufficiently high to produce a uniform solution or melt (i.e. prefusion). Preferably, the temperature to which the batched material is heated to produce a uniform solution of the slag constituents is only slightly above the plastic deformation point and thereafter heating is discontinued. Care should be taken to avoid overheating the batched material, as temperatures substantially above about 2,700F will volatilize certain of the important ingredients of the slag, such as the alkali metal oxides, fluorides and boron oxides. The resulting prefusion or prefused slag on cooling can be used asfritted but preferably is first ground or otherwise finely divided to a powder-like consistency, preferably having a maximum particle size of about mesh (U.S. Std.) and preferably having at least 50 percent by weight passing through a 100 mesh screen (U.S.Std.). The slag material without further treatment or special handling can be introduced into a mold in solid particulate form to provide a slag layer on the surface of the molten metal in a continuous casting mold. If desired, however, the particulate slag composition can be uniformly mixed with finely divided carbon material, such as graphite, lamp black, coke or charcoal, in an amount between about 1 percent and about 10 percent by weight of the slag composition.

In use the as-fritted or the powdered slag composition or slag-graphite mixture can be added to a continuous casting mold directly on the surface of the molten steel to form a layer of molten slag having a thickness of at least about one-half inch and preferably a layer having a thickness of from about 1 inch to about 2 inches. The layer of slag is maintained throughout the continuous casting process by periodic or continuous additions of the powdered slag mixture. When casting an aluminum-killed low carbon steel slab having dimensions of 8 inches X 37 inches at a casting rate of about inches per minute, a typical slag of this invention is used at a rate of about 1.3 lbs. per ton of steel cast.

In order to further illustrate the present invention, the following specific examples are provided without, however, limiting the invention to the specific materials or the proportions used in the specific examples.

EXAMPLE 1 A uniform fusion mixture was prepared by heating the specified oxides and fluorides batched in the following proportions:

Na O 15 percent by wt. SiO 21 percent by wt. C110 23 percent by wt. CaF 17 percent by wt. LiF 3 percent by wt.

B 0 21 percent by wt.

The solidified slag was reduced by grinding to form slag granules having a maximum particle size of about 20 mesh (U.S.Std.) with about 50 percent of the granules passing through a mesh screen (U.S.Std.

The slag had a measured flowidity of 10.9 inches, a calculated flowidity of 11.5 inches, a measured plastic deformation point of 1,500F, a calculated plastic deformation point of 1,493F, and an alumina (A1 0 solubility of 32 per cent by weight. The measured flowidity and plastic deformation point were determined by the herein described procedures and the calculated values were obtained by applying the herein disclosed equations using the chemical composition of the batched slag mixture.

The apparatus used to determine the flowidity of the slag compositions of the present invention and shown in FIG. 2 of the accompanying drawing is a split mold 10 comprising half mold sections ll, 12 cast or machined of low carbon steel. The mold sections l1, 12 when operatively assembled, as by clamping together the two half mold sections ll, 12, form a vertically disposed funnel or conical portion 13 having an internal diameter of about 3 inches and an external diameter of 3% inches at the upper end and an internal diameter of five-eighths inch at the lower or discharge end thereof. The internal lateral surface of the conical portion 13 forms an angle of 39.5 with the vertical axis of the conical portion 13. A conical recess 14 formed by the conical portion 13 is adapted to receive the test sample of molten slag. A rectangular base section 15 connects with the lower end of the conical portion 13 and defines a cylindrical well 16 having a diameter of five-eighths inch and a depth of about three-fourths inch. The base section 15 has rectangular section 17 extending horizontally from one side thereof with an axial bore or conduit 18 extending therethrough and having a circular cross section one-fourth inch in diameter and a length of about 20 inches. The conduit 18 intersects the lateral cylindrical well adjacent the lower end of the well 16 and provides an outlet passage for the slag test sample introduced into the conical recess 14 which serves as a funnel to feed the test sample into the conduit 18 until the sample solidifies in the conduit 18.

The flowidity" of the slag as the term is used in the specification and claims is the distance in inches which the slag composition flows through the conduit 18 before solidifying when 200 gms. of the prefused slag at a temperature of 2,600F is rapidly poured into the conical recess 14. The pouring technique and timing of the test procedure should be standardized to obtain con-' sistent results. The temperature of the mold, within normal operating limits, has little effect on the measured flowidity."

The plastic deformation point of the slag as the term is used in the specification and claims is determined by heating about a 20 gram piece of the solidified prefused slag which is obtained from the above flowidity test at a rate of about 50 F per minute and pressing a graphite rod (or an alumina or other suitable rod) against the piece of slag at frequent intervals as the temperature rises, and the temperature at which the piece of slag plastically deforms under a slight pressure is the plastic deformation point.

The alumina solubility of the slag as the term is used in the specification and claims is determined by placing 50 gms. of the prefused slag in a pure alumina crucible approximately 3 inches in diameter, about 1 inch high with a wall thickness of about one-eighth inch, heating to a temperature of 2,600F and holding at 2,600F for a period of 1 hour. The slag is analyzed for alumina content before and after the test procedure, and the increase in weight percentage of alumina in the slag is taken as the alumina solubility" of the slag.

The particulate slag composition after mixing with percent by weight finely divided flake graphite (l00 mesh, U.S. Std. and 88-94 percent carbon) was used in a continuous casting apparatus, was non-fuming and produced excellent casting results.

EXAMPLE 2 A uniform fusion mixture was prepared by heating the specified oxides and fluorides batched in the following proportions:

Na,O 22.0 percent by wt. CaF, 24.0 percent by wt. SiO, 33.4 percent by wt. CaO l 1.0 percent by wt. LiF 7.0 percent by wt. K 0 2.6 percent by wt.

The solidified melt was then reduced by grinding to form slag granules having a maximum particle size of about 20 mesh (U.S. Std.) with about 50 percent passing through a 100 mesh screen (U.S.Std.).

The foregoing slag composition had a measured flowidity of 8 inches, a calculated flowidity of 9 inches, a measured plastic deformation point of 1,490F, a calculated plastic deformation point of l,520F and an alumina solubility of 41 per cent by wt. The flowidity, plastic deformation point, and alumina solubility of the slag composition were determined as described in Example l.

The slag composition of Example 2 after mixing with 5 percent by weight finely divided flake graphite (l-0O mesh, U.S. Std. and 88-94 percent carbon) was used in a continuous casting apparatus for casting aluminum killed low carbon steel without fuming and produced excellent casting results.

I EXAMPLE 3 A uniform fusion mixture was formed by heating the specified oxides and fluorides to form a uniform melt having the following chemical composition:

B 0; l8.4 percent by wt. SiO 23.0 percent by wt. CaO 28.6 percent by wt. Na O l8.l percent by wt. CaF 9.5 percent by wt. LiF 2.4 percent by wt.

The solidified substantially homogeneous slag was then reduced by grinding to form slag granules having a maximum particle size of about 20 mesh (U.S. Std.) with at least 50 percent thereof passing through a 100 mesh screen (U.S. Std.).

When the foregoing slag composition was tested as described in Example 1, the measured flowidity was 10.4 inches compared with the calculated flowidity of 10.7 inches, and the measured plastic deformation point was 1,500F compared with the calculated plastic deformation point of 1,539F.

EXAMPLE 4 A uniform melt was formed by heating a mixture of oxides and fluorides to form a prefused slag having the following chemical composition:

B 01 l8.0 percent by wt. SiO 26.4 percent by wt. C210 3 l .2 percent by wt. Na O 12.3 percent by wt. CaF 8.5 percent by wt. LiF l.4 percent by wt. M 0: 2.2 percent by wt.

The solidified fused slag was then reduced by grinding-to form a powdered slag having a maximum particle size of 20 mesh (U. S. Std.) with at least 50 percent passing through a lOO mesh screen (U. S. Std.)

The foregoing slag composition when tested as in Example 1 had a measured plastic deformation point of l,540F, compared with a calculated plastic deformation point of l,623F, and a measured flowidity of l0.2 inches compared with a calculated flowidity of 9.7 inches. The alumina solubility of the slag was 37 per cent by weight.

EXAMPLE 5 Na,0 27.2 percent by wt. SiO, 52.8 percent by wt. CaF, 17.0 percent by wt. LiF 3.0 percent by wt.

deformation point of 1,574F. The alumina solubility of 5 the slag composition was 30 percent by weight.

EXAMPLE 6 A prefused mixture prepared by heating the following specified oxides and fluorides batched in the following proportions:

B 4.0 percent by wt. N3 0 l0.0 percent by wt. CaF 30.0 percent by wt. CaO 25.0 percent by wt. SiO, 23.0 percent by wt. LiF 8.0 percent by wt.

The calculated flowidity of the foregoing slag composition is l3.3 inches and the calculated plastic deformation point is l,6lF.

An important feature of the slag composition of the present invention is its ability to remove from the molten steel substantial amounts of the oxides of aluminum which are present in many steels, such as aluminum killed steels, and which tend to form objectionable inclusions in the surface of steel continuous castings. When casting steel which contains oxides of aluminum, such as A1 0 it is important to prevent aluminum oxide particles being included in the surface of the continuous casting by providing a slag which will dissolve or otherwise remove aluminum oxides. It has been found that by combining a limited amount of boron producing compounds or fluoride salts with the other components of the slag composition of the present invention, it is possible to dissolve and retain in the slag large amounts of objectionable aluminum oxides, particularly aluminum trioxide. And, by maintaining the combined weight per cent of the B 0 CaF LiF in the batched slag composition above per cent by weight, the slag will dissolve at least per cent by weight alumina based on the weight of the slag. A preferred slag of the present invention, for example, when placed in contact with solid aluminum oxide (A1 0 and maintained at 2,600F for a period of one hour in contact therewith will pick up about 37 per cent by weight aluminum oxide (M 0 )v The relative solubility of aluminum oxide (A1 0 in several slag compositions is shown in the following TABLE I:

(1) Approximate composition 64% Si0,, 36% Na,0.

(2) Approximate composition SiO 20% Na o. 10% CaO The data of TABLE I show that slags which do not contain any lithium fluoride, calcium fluoride or boron trioxide such as the sodium disilicate and window glass slag, have a low solubility for alumina, whereas a slag which contains lithium fluoride, calcium fluoride and boron trioxide exhibits a high solubility for alumina.

A 5 pound quantity of powdered graphite was added and uniformly mixed with pounds of each of the slags of Example 3 and Example 4 in particulate form to provide slag-graphite mixtures for use in the continuous casting of steel. Flake graphite having a particle size which passed through a 100 mesh screen (U.S. Std.) and a carbon content of 88-94 percent by wt. was used.

When. the slag-graphite mixture of Example 3 was used in a continuous casting slab mold 8 inches by 37 inches to produce a 100 ton heat of conventional aluminum killed low carbon steel without causing fuming or producing objectionable contamination, it was found from heat transfer studies that 28.3 BTUs were removed per pound of steel in the continuous casting mold, as contrasted with 24.4 BTU s per pound of steel casting with a commercial slag composition having the following approximate composition: Si0 37 percent, Al O 20 percent, CaO 20 percent, MgO 5 percent and CaF 12 percent by weight. It has generally been found that the slag compositions of the present invention when used for the continuous casting of steel increase the heat transfer between the steel and the mold about 15 percent above that obtained with the use of traditional rape seed oil or with prior commercial continuous casting mold slags. Moreover, the slags of the present invention also increase the heat transfer in the secondary cooling zone after the casting leaves the mold by about 25 percent over any of the prior continuous casting practices. The improvements in both mold and secondary heat transfer make possible substantially faster casting speeds than with prior slags and, therefore, facilitate greatly increasing the tonnage output from a given continuous casting installation.

Continuous steel castings were made from a steel having the following heat analysis: 0.05 C, 0.34 Mn, 0.017 P, 0.031 S, 0.01 Si, 0.054/0.042 Al in the above described continuous casting mold using (1) the slag of Example 4 and (2) the above mentioned commercial slag, and the resulting continuous castings were processed into cold rolled steel sheet by conventional means. When the latter sheets were carefully inspected for surface defects, it was found that the steel sheets made from the castings produced with the slag of Example 4 were from 92 to [00 percent free of surface defects with only 8 percent of the steel sheets having minor defects and none of the steel sheets having major defects. When the cold rolled steel sheets produced with the above mentioned commercial slag composition were examined, they were found to have 86 to 100 percent with minor defects, 3 to 14 percent had serious surface defects, and there were no defect-free steel sheets.

Cold rolled steel sheets were also produced by continuously casting a steel having the foregoing analysis in the above described mold using (a) the slag of Example 4 mixed with 5 percent by weight flake graphite and (b) a 100 percent borax slag mixed with 5 percent by TABLE II Sheet inspection Results,

Defect Minor Serious Mold Slag Used Free Defect Defect Example N0. 4+5% Graphite 97.0 3.0 100% Borax+% Graphite 80.5 8.5 l l The slag compositions of the present invention exhibit a combination ofproperties which makes the slags particularly suited for producing high quality continuous castings from steels containing appreciable amounts of aluminum, while avoiding the objectionable properties, such as objectionable fuming, decrease in flowidity at high temperatures and contamination of the steel, of the prior continuous casting slags. Of the herein disclosed compositions, the composition of specific Example 1 is a preferred composition where it is permissible for the slag composition to contain boron oxides, and the slag composition of specific Example 2 is a preferred composition where the slag composition is free of boron oxides.

It should also be understood that while the relationship between the several chemical elements comprising the slag composition of the present invention is very complex in the fused state, when the oxide and fluoride containing materials are mixed in the batched formulation so that the batched material contains the specified oxides and fluorides within the percentage ranges specified and when the fused slag also concurrently exhibits the flowidity, plastic deformation point and alumina solubility properties within the ranges specified therefore, the resulting slag composition possesses the improved casting properties disclosed herein for the continuous casting of steel resulting in better continuous steel castings and in improved rolled sheet material having fewer surface defects. And, if the slag composition does not possess in combination each of the essential criteria specified herein, the slag composition will not have the desired improved casting properties. However, where the slag composition does not possess, for example, flowidity or plastic deformation point properties falling within the ranges specified herein, it is a simple matter for one skilled in the slag art to adjust the oxide or fluoride content of the batched material to bring the value within the herein specified ranges by utilizing the knowledge of those skilled in the slag art and the teaching of the present disclosure.

We claim:

1. In a process of continuous casting of steel in which a pool of molten steel is maintained in the upper end of an open-ended continuous casting mold and continuously cooling the mold to solidify molten steel in at least the lower end portion thereof, the improvement comprising; providing on the surface of said pool of molten steel a layer of a synthetic slag consisting essentially of oxides and fluorides, said slag having on an asbatched basis a chemical analysis for fusible oxides and fluorides as follows:

1'. silicon dioxide within a range of 10 per cent to 55 per cent by weight,

calcium fluoride within a range of 5 per cent to 40 per cent by weight,

3. an alkali metal compound selected from the group consisting of sodium oxide and potassium oxide within the range of 5 per cent to 30 per cent by weight,

. a lithium compound selected from the group consisting of lithium oxide and lithium fluoride within the range of0.5 per cent to 15 per cent by weight,

5. calcium oxide in an amount up to 40 per cent by weight,

. boron trioxide in an amount up to 30 per cent by weight, and

7. having the combined amounts of boron trioxides. calcium fluoride and lithium fluoride in excess of 15 per cent by weight,

and said oxides and fluorides combined in said slag to provide said slag with a plastic deformation point between about 1,100F and about l,700F in accordance with the following question:

Plastic Deformation and said oxide and fluorides also being combined in said slag to provide said slag with a flowidity between about 2 inches and about 16 inches in accordance with the following equation:

Flowidity (inches) =').4+0.02 wt. oeoma o) +0.05 wt. (B 0 +0.10 wt. (CaO+CaF,) +0.32 wt. (LiF+Li,O) 0.20 wt, (sio,)

and said slag having an alumina solubility in excess of 20 per cent by wt. of said slag.

2. A process of continuous casting as in claim 1, wherein said molten steel is an aluminum killed low carton steel.

3. A process of continuous casting as in claim 1, wherein said oxide and fluorides prior to addition to said continuous casting mold are heated to form a uniform melt and solidified to form a frit.

4. A process of continuous casting as in claim 3, wherein said frit is ground into granules prior to addition to said continuous casting mold.

5. A process of continuous casting as in claim 4, wherein said granules are mixed with about 1 to 10 percent by weight finely divided carbon particles prior to addition to said continuous casting mold.

13 1. 6. A process of continuous casting as in claim 1, 9. A process of continuous casting as in claim 1, wherein said slag is maintained on the surface of said wherein said oxides and fluorides have the following as pool of molten metal as a molten layer having a baged chemical analysis:

thickness of from one-half inch to 2 inches.

7. A process of continuous casting as in claim 1, 5

wherein said oxides and fluorides have the following as 18: g g-g W ay i 2 .percen yw. batched chemical analysis. cao 3 L2 percent by wt Na O 12.3 percent by wt. 1 15 percent by wt. C aF 8.5 percent by wt. SiO 21 percent by wt. LIF 1.4 percent by wt. C210 23 percent by wt. 7 A1 0 2.2 percent by wt. CaF, 17 percent by wt. UP 3 percent by wt. 13,03 21 percent by wt. 10. A process of continuous casting as in claim 1,

wherein said oxides and fluorides have the following as 8. A process of continuous casting as in claim 1, batched chemicalanalysisz wherein said oxides and fluorides have the following asbatched chemical analysis:

B 0 4.0 percent by wt.

Na 0 10.0 percent by wt. Na, 0 22.0 percent by wt. Cal} P y CaF 24.0 percent by wt. CaO 25.0 percent by wt. SiO 33.4 percent by wt. SiO 23.0 percent by wt. CaO 11.0 percent by wt. LiF 8.0 percent by wt. LiF 7.0 percent by wt. K 0 2.6 percent by wt.

QERTEFEQATE 3F QfiiiREQTlQN Patent No- 3 704 ,744 Dated December 5, 1972 Inventor(s) Halley et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 12 Claim 2, line 3, "carton" should read -carIb on-- Signed and sealed this 22nd day of May 1973.

(SEAL) Attest:

EDWARD PLFLETCI-LERJR. ROBERT GOTT'SCHALK Attesting Officer Commissioner of Patents 

2. A process of continuous casting as in claim 1, wherein said molten steel is an aluminum killed low carton steel.
 2. calcium fluoride within a range of 5 per cent to 40 per cent by weight,
 3. A process of continuous casting as in claim 1, wherein said oxide and fluorides prior to addition to said continuous casting mold are heated to form a uniform melt and solidified to form a frit.
 3. an alkali metal compound selected from the group consisting of sodium oxide and potassium oxide within the range of 5 per cent to 30 per cent by weight,
 4. a lithium compound selected from the group consisting of lithium oxide and lithium fluoride within the range of 0.5 per cent to 15 per cent by weight,
 4. A process of continuous casting as in claim 3, wherein said frit is ground into granules prior to addition to said continuous casting mold.
 5. A process of continuous casting as in claim 4, wherein said granules are mixed with about 1 to 10 percent by weight finely divided carbon particles prior to addition to said continuous casting mold.
 5. calcium oxide in an amount up to 40 per cent by weight,
 6. boron trioxide in an amount up to 30 per cent by weight, and
 6. A process of continuous casting as in claim 1, wherein said slag is maintained on the surface of said pool of molten metal as a molten layer having a thickness of from one-half inch to 2 inches.
 7. having the combined amounts of boron trioxides, calcium fluoride and lithium fluoride in excess of 15 per cent by weight, and said oxides and fluorides combined in said slag to provide said slag with a plastic deformation point between about 1,100*F and about 1,700*F in accordance with the following question: Plastic Deformation (Point (*F) ) 1540- 1 wt. % (K2O+Na2O)-14 wt. % (B2O3)-54 wt. % (LiF+Li2O)+10 wt. % (CaO+CaF2)- 1 wt. % (SiO2), and said oxide and fluorides also being combined in said slag to provide said slag with a flowidity between about 2 inches and about 16 inches in accordance with the following equation: Flowidity (inches) 9.4+0.02 wt. % (K2O+Na2O) +0.05 wt. % (B2O3)+0.10 wt. % (CaO+CaF2)+0.32 wt. % (LiF+Li2O)-0.20 wt. % (SiO2) and said slag having an alumina solubility in excess of 20 per cent by wt. of said slag.
 7. A process of continuous casting as in claim 1, wherein said oxides and fluorides have the following as batched chemical analysis: Na2O 15 percent by wt. SiO2 21 percent by wt. CaO 23 percent by wt. CaF2 17 percent by wt. LiF 3 percent by wt. B2O3 21 percent by wt.
 8. A process of continuous casting as in claim 1, wherein said oxides and fluorides have the following as-batched chemical analysis: Na2 O 22.0 percent by wt. CaF2 24.0 percent by wt. SiO2 33.4 percent by wt. CaO 11.0 percent by wt. LiF 7.0 percent by wt. K2O 2.6 percent by wt.
 9. A process of continuous casting as in claim 1, wherein said oxides and fluorides have the following as batched chemical analysis: B2O3 18.0 percent by wt. SiO2 26.4 percent by wt. CaO 31.2 percent by wt. Na2O 12.3 percent by wt. CaF2 8.5 percent by wt. LiF 1.4 percent by wt. Al2O3 2.2 percent by wt.
 10. A process of continuous casting as in claim 1, wherein said oxides and fluorides have the following as batched chemical analysis: B2O3 4.0 percent by wt. Na2O 10.0 percent by wt. CaF2 30.0 percent by wt. CaO 25.0 percent by wt. SiO2 23.0 percent by wt. LiF 8.0 percent by wt. 